In recent years, in the automotive industry, the introduction of devices aimed at increasing safety is being actively promoted. Such devices includes airbag systems, which have been developed and mounted on vehicles. An airbag system is a system which at the time of an automotive collision expands an airbag with gas or the like between a passenger and the steering wheel, the instrument panel, or other object before the passenger strikes these objects in order to absorb the kinetic energy of the passenger and reduce their injuries. In the past, airbag systems have used explosive chemicals, but in recent years, airbag systems which use a high pressure filler gas have been developed and are being increasingly used.
The above-described system using a high pressure filler gas always maintains a gas at a high pressure, and at the time of a collision, the high pressure gas is discharged into an airbag all at once. Accordingly, stress is applied to a steel tube used as an accumulator for the high pressure gas at a high strain rate in an extremely short period of time. Therefore, in contrast to a conventional pressure cylinder or a line tube which is a mere structural component, the above-described steel tube is required to have a high strength and excellent bursting resistance as well as good dimensional accuracy, workability, and weldability.
Recently, ultrahigh strength seamless steel tubes having a tensile strength greater than 1000 MPa are being used as accumulators for airbag systems in order to guarantee a high bursting pressure even when the wall thickness and the weight of the accumulators are reduced. For example, whereas the bursting pressure of an accumulator made from a seamless steel tube having an outer diameter of 60 mm and a wall thickness of 3.55 mm is only around 100 MPa when TS is 800 MPa, the bursting pressure increases to 130 MPa when TS becomes 1000 MPa. Furthermore, when the outer diameter of a steel tube for an accumulator of an airbag and the required bursting pressure are fixed, it is possible to decrease the wall thickness by around 20%.
An accumulator also needs to have excellent low temperature toughness so that the accumulator will not undergo brittle fracture and cause a secondary accident at the time of a collision even in cold regions.
From this viewpoint, a seamless steel tube for an accumulator has been imparted a high strength and high toughness by quench hardening and tempering. Specifically, after an accumulator has undergone the below-described diameter reduction, it is required that it have sufficient low temperature toughness in a temperature range of −60° C. or below.
An airbag accumulator is typically formed by cutting a seamless steel tube which is a blank tube to be processed to a prescribed length to obtain a short tube, subjecting at least one end of the short tube to diameter reduction by working such as press forming or spinning (this step is referred to as bottle forming), and finishing the short tube to a final shape necessary for mounting on an initiator or the like. Accordingly, in order to guarantee operation of an accumulator for an airbag, the toughness of the seamless steel tube used as a blank tube is sometimes inadequate. This is because the toughness of the bottle-shaped portion of the resulting accumulator decreases due to the final working for diameter reduction, whereby cracks may develop in that portion when a high pressure is applied to the accumulator. Taking into consideration such a decrease in toughness, a seamless steel tube used in the manufacture of airbag accumulators needs to have toughness at a lower temperature than the temperature of the environment of use of an accumulator.
From this standpoint, a seamless steel tube used to constitute an accumulator is required to have elongation of at least 10%, a tensile strength of at least 1000 MPa, and low temperature toughness such that fracture appearance is ductile in a Charpy impact test at −80° C. and preferably at −100° C. (namely, it has low temperature toughness such that vTrs100 is −80° C. or below and preferably −100° C. or below).
Patent Document 1 is an example of prior art relating to a seamless steel tube for an airbag system having a high strength with a tensile strength of at least 1000 MPa and high toughness. Patent Document 1 proposes a process for manufacturing a seamless steel tube for airbags comprising producing a seamless steel tube using a steel having a chemical composition in a certain range, subjecting the seamless steel tube to cold drawing to obtain a steel tube with predetermined dimensions, quench hardening the steel tube after heating to a temperature in the range of at least the Ac3 transformation point to at most 1050° C., and performing tempering of the tube at a temperature in the range of at least 450° C. to at most the Ac1 transformation point.
It is purported in that document that this process can provide a seamless steel tube having excellent workability and weldability at the time of manufacture of an inflator for an airbag, which has a tensile strength of at least 900 MPa as an inflator, and which has high toughness such that the steel tube exhibits ductility when it is cut in half and subjected to a drop weight test at −60° C. However, in order to obtain such a strength and toughness, it is necessary to employ a steel with composition containing a large amount of Cr, so this process is expensive.
Patent Document 2 discloses that if quench hardening by high frequency induction heating is used, it is possible to manufacture a seamless steel tube for an airbag system having a high strength with a tensile strength exceeding 1000 MPa and a high toughness due to grain refinement caused by the rapid heating.
With that technique, after a seamless steel tube is manufactured using a steel having a chemical composition in a prescribed range, the seamless steel tube is then subjected to cold drawing to obtain a steel tube with prescribed dimensions, then to heating to 900-1000° C. at a heating rate of at least 10° C. per second, to quenching, and to tempering at a temperature not higher than the Ac1 transformation point. This technique is intended to achieve a high toughness such that ductility is exhibited in a burst test at −80° C. or below. In Patent Document 2, a specific example is given of heating at a rate of 20° C. per second for quench hardening. However, taking into consideration industrial productivity, it is desirable to perform rapid heating in a shorter period of time and to shorten the holding time at the temperature which is reached. When performing heat treatment with rapid heating in a short period of time and a short holding time, the temperature which is reached may locally fall below the Ac3 point due to variations in the heating temperature. Therefore, if possible, it is desirable to set the heating temperature on the high side. However, with high frequency induction heating, due to rapidness of heating, the problem of overshooting in which heating temperature is beyond the set temperature may occur. For this reason, it is necessary to take into consideration the case in which the temperature which is reached during high frequency induction heating for quenching exceeds 1000° C. However, Patent Document 2 says nothing about this problem which occurs during mass production. Rather, it states that a temperature in excess of 1000° C. leads to coarsening of gamma (γ) grains and a decrease in toughness.
Patent Document 3 discloses examples in which high frequency induction heating is employed for quench hardening. However, as shown in Table 3 of the examples of that document, only heating for a short period of time in the range of 900-1000° C. is contemplated. Thus, the technique disclosed in this document has the same problems as discussed for Patent Document 2.
Patent Document 4 discloses quench hardening by high frequency induction heating, but the examples are the result of heating in the range of 920-940° C., so the technique disclosed in this document has the same problems as discussed for Patent Document 2.